Late Roman Unit Sizes

[Nathan Ross]

am I right in still believing that the 'original' Constantinian Field Army with it's 5 cavalry vexillatio's; 5 Legios Palatina; and 10 Auxilia Palatina that I saw listed lo' those many years ago in Barker's Armies and Enemies series - has a detailed source somewhere?

Not as far as I know. No army of the era is described in any great detail - excepting some of the legion detachments apparently with Galerius in Egypt. I assume Barker has come up with a reasonable estimate based on total army sizes from different sources, but as we still can't decide on the size of either a legion or an auxilium at this date it's pure guesswork.

Eusebius gives Constantine 20,000 men at Cibalae (AD316/17), while Panegyric XII claims he invaded Italy in AD312 with 'fewer than 40,000' men. However, Zosimus (II.15) says that the emperor had 98,000 by the time of Milvian Bridge. The latter figure seems incredible.

Field armies of between 15,000 and 30,000 seem most common during this period, but with the contrary claims and exaggerations of different writers it's hard to more definite than that.